Writing data into serverless db from AWS lambda is extremely slow

I have created a MongoDB Atlas serverless db in the AWS region of eu-west-1.
Then I was trying to write data into this db from a AWS lambda function. This function is also in the AWS region of eu-west-1.

The connection between my db and my lambda function was built via Private Endpoint.

The average data writing speed was 35 KB/s, which was so slow. I wonder what may have caused that?

I recommend opening a support case